For purposes of this application, magnetic or other tape rolls and reels, film on reels, printing ribbons on spools and other flat, rolled strips of material are all referred to as "tape".
The typical tape cartridge housing is a boxlike container having a bottom half--a base--constructed of metal. For example, U.S. Pat. Nos. 4,172,569 to Newell and 4,380,032 to Pfost show a metal base plate onto which the vast majority of inner workings are attached. The base plate is machine cut to the desired size and shape. Dimensionally the cartridge conforms to ANSI (Americal National Standard Institute) standard number X3B1/625. A number of stationary tape guide members and a greater number of pins are then pressed into the plate. The pins are used for a variety of purposes, the most important of which are to hold the tape wound supply and take-up hubs and to retain the drive belt guide members at fixed locations.
The base plate is then made of metal for reasons of durability and stability. Durability of the plate is a major concern because of the constant pressure on the pins as the tape hubs and guide members rotate upon them. It is well known in the tape recording art that a myriad of problems will occur if the tape is not wound upon the hubs in uniform tension or if pins, guides or rotating members are not maintained in suitable alignment. For example, if a tape supply hub contains a high tension point somewhere on the supply of tape, relaxation takes place when that point is exposed during playback. This causes the tape undergoing relaxation to become somewhat shorter and somewhat thicker, thereby momentarily slowing the tape's motion across the magnetic head. The result is variously known as time displacement error (TDE), time base error (TBE), jitter, wow, or flutter. To reduce the possibility of such error, it is important that the base plate always retain its shape. Another example is tape misalignment, misregistration, and possible damage that can be caused by thermally induced cartridge component movements. Thus, the plate is typically made of metal.
The top half of the tape cartridge, on the other hand, is made of a less expensive, more easily formed material having a different coefficient of thermal expansion. U.S. Pat. Nos. 4,172,569 and 4,380,032, mentioned above, show the use of a plastic top. Transparent plastic is often a desirable material because it allows an operator to peer into the cartridge if he or she so desires and it permits a manufacturer to make improvements involving optics without changing the design of the cover.
The prior art teaches the use of screws to attach the plastic top to the metal base plate. Since the two materials have different coefficients of thermal expansion, one of the sides will warp, or even buckle, as the cartridge heats during use. If the plastic top buckles, there may be difficulty taking the cartridge out of the tape drive. In addition, warp will distort any optical signals being passed through the plastic top. Finally, the possibility of cover buckle requires the manufacturer to leave larger clearances between the cover and any moving parts within the cartridge. Because the ANSI standard limits the size of the cartridge, the need for minimizing clearances may be critical to developing future cartridge improvements.
Any warping of the plastic cover or the base plate will increase the chances of tension variations or other mishandling as tape winds from one tape hub to the other. As mentioned above, changes in tape tension cause distortion when the tape is played back. Yet, high speed tape movement and mechanical power transferred to the cartridge tend to create heat within the cartridge which is removed by conduction or convection into the cartridge walls with more heat being conducted to the metal base, as a result of the pins being firmly anchored to the base. This unequal transfer of heat as well as the difference in coefficients of thermal expansion, cause the cover dimensions to change by amounts that differ from the base. This thermally induced differential dimension change is the source of the problem described above.
The problem solved by the present invention is similar to the one addressed in U.S. Pat. No. 4,571,789 to Morioka. However, in that patent pins with specially shaped heads in two directions and reduced necks extend from a cartridge top and fit into undercut holes in a cartridge base plate. While the resulting cartridge has merit for overcoming thermal stress in a cartridge, a simpler solution is needed.
An object of this invention is to provide for joining closures and bases having different coefficients of thermal expansion which permits expansion without a warp or buckle to either member.